A circle has a sector with area $\dfrac{5}{4}\pi$ and central angle $18^\circ$. What is the area of the circle? ${25\pi}$ $\color{#9D38BD}{18^\circ}$ ${\dfrac{5}{4}\pi}$
Solution: The ratio between the sector's central angle $\theta$ and $360^\circ$ is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{360^\circ} = \dfrac{A_s}{A_c}$ $\dfrac{18^\circ}{360^\circ} = \dfrac{5}{4}\pi \div A_c$ $\dfrac{1}{20} = \dfrac{5}{4}\pi \div A_c$ $A_c \times \dfrac{1}{20} = \dfrac{5}{4}\pi$ $A_c = \dfrac{5}{4}\pi \times 20$ $A_c = 25\pi$